The present invention relates to an apparatus for heat treating mineral ore material and, in particular, to systems which have a series-flow arrangement, a preheater having at least two stages for drying and preburning material, a kiln and a cooler.
U.S. Pat. No. 2,466,601 discloses a system in which minerals are deposited on a traveling grate and carried through a drying chamber, a preburning chamber, and then are deposited in a rotary kiln for final burning. Hot gases in the kiln heat materials to high temperatures and then pass from the kiln to preburn and then dry the material before passing up a stack at relatively low temperatures. A number of methods for burning a number of materials have been successfully commercialized with such equipment. Iron ore, limestone, and limestone with clay are a few examples of such materials; and examples of such methods are disclosed not only in U.S. Pat. No. 2,466,601 but also in U.S. Pat. No. 2,580,235; U.S. Pat. No. 2,925,336; U.S. Pat. No. 3,110,483; U.S. Pat. No. 3,110,075; U.S. Pat. No. 3,313,534; U.S. Pat. No. 3,416,778; U.S. Pat. 3,653,645; and U.S. Pat. No. 3,671,027. One problem involved in the operation of a system as disclosed in the aforementioned U.S. Pat. No. 2,466,601 is that of obtaining proper thermodynamic balance of heat inputs among the drying, preburning and final heating stages. This problem arises because for each material there are three requirements that establish desired temperatures within such systems. The first requirement is that for each material there is a known or ascertainable heat input and temperature level to which the material must be finally heated in the rotary kiln. The second requirement is that each material also has a known or ascertainable temperature level and total heat input that is necessary to achieve the desired preburn. The material is exposed to much higher temperatures in the kiln. The third requirement is that each material also has a known or ascertainable desired maximum gas temperature for drying the material so that water vapor is not produced so rapidly that the material breaks into particle sizes so small that excessive dust is created. Thus, a material requiring a relatively low drying temperature to prevent particle break-off will require a relatively large volume of gas to completely dry the material, and a material that can tolerate a relatively high drying temperature may require a relatively small volume of drying gases.
Although some materials burn with partial exothermic reactions, it is nevertheless true for all materials that the temperature and volume of the gases that perform the final heating, and therefore determine the size of the kiln, are determining factors as to the temperature and volume of gases discharged from the kiln for preburning and drying material on the grate. Therefore, the degree to which the second requirement is achieved affects the degree to which the first and third requirements can be achieved.
A problem of proper thermodynamic balance between the drying, preburning and final heating stages is created because the gas flow begins with a specific volume of preheated gas from the cooler mixing with burning fuel in the kiln to meet the first requirement, and it is difficult with prior art processes to be sure that the volume and temperature of the gases finally reaching the drying chamber are what is needed to meet the third requirement without providing the kiln that is oversized and without wasting heat from one or more stacks to atmosphere.
The aforesaid U.S. Pat. No. 3,313,534 discloses a system comprising a two-stage cooler with preheated air from the first cooler stage passing into the kiln and air from the second stage discharged to atmosphere as waste heat. An auxiliary burner over the grate and a bypass are provided for some of the gas from the kiln to bypass directly to the drying chamber. With such a system, a regulated quantity of kiln gas that has not passed through material in the preheat chamber may be mixed with gas that has passed through the material in the preheat chamber and the mixture passed through material in the drying chamber. Although this system achieves proper thermodynamic balance, it requires more fuel and a kiln which is larger in diameter than is required for a system according to the present invention for a reason that will appear and be explained as this description of prior art proceeds.
U.S. Pat. No. 2,214,345 and the aforesaid U.S. Pat. No. 2,580,235 disclose bypassing preheated air from the cooler around the kiln and preburn chambers to a drying chamber, and U.S. Pat. No. 2,580,235 additionally discloses one embodiment in which kiln gas can be also bypassed to a drying chamber without passing through material in the preburn chamber. However, such systems also require oversized kilns for a reason that will now be explained. Oversized kilns are required because at start-up and before hot pellets reach the cooler, the cooler provides no heat and all heat needed for the chambers over the grate must come from gases passing through the kiln. The kiln must, accordingly, be sized to accommodate the greater temporary gas flow until hot pellets reach the cooler where some of their heat can be recovered and bypassed around the kiln to the chambers over the grate.
The aforesaid U.S. Pat. Nos. 2,214,345; 3,416,778; and 3,653,645, in addition to U.S. Pat. No. 3,513,534, also disclose burners over a grate for aiding to achieve proper preburning on a grate and ahead of the kiln. However, the burner over the grates in U.S. Pat. No. 2,214,345 does not in any way affect the temperature or volume of gases used for drying, and therefore offers no solution to the problem of material being insufficiently dried and entering the second treatment chamber too wet, during start-up operation before hot material has reached the cooler where thermal energy can be transferred to gases and used for drying. The burners over the grate in U.S. Pat. Nos. 3,313,534; 3,416,778; and 3,653,645 can affect the temperature of gases used for drying, but after pellets begin to pass from the drying chamber into the preburning chamber, the preburning operation utilizes heat which is therefore no longer available for the drying operation, and such systems, therefore, also require oversized kilns or overfiring the burners over the grate. Overfiring the above grate burners in the preburn chamber merely to provide excess heat for drying operations is undesirable because so doing can heat the upper layers of pellets in the preburn chamber beyond the preburn desired before pellets begin to tumble through the kiln.
In U.S. Pat. No. 3,782,888, provisions are made for recouping heat from the cooler by preheating air in a second stage of the cooler, bypassing such preheated air around the kiln, at least around the preburn chamber over a grate which is adjacent the kiln. The invention is characterized by the provision of a heater that can be operated to inject thermal energy into the air stream from the second stage of the cooler that bypasses the kiln and is used for drying operations, at least during start-up when hot material has not yet progressed to the second stage of the cooler. The air heater for cooler gases bypassing the kiln to a drying chamber can, according to the patent, be utilized with a bypass discharge gas around the preburn zone and with two-stage drying in a manner that will provide for a smaller sized kiln.
The present invention is directed to the problem of reducing kiln size and fuel requirements relative to tonages of material treated and providing controlled thermodynamic balance in such systems by the utilization of kiln bypass gases for operation according to a method that will be described. In the present process, the system starts at a reduced capacity until such time as hot pellets enter the cooler. The bypassed kiln gases are utilized for thermally balancing the system operating conditions until heat from the cooler is available. The combination of cooler heat and burner heat results in lower process gas required through the kiln. As a result, a much smaller kiln can be provided. The reinforced direct-fired grate preheater kiln system to be described can supply the additional heat to the grate preheater for accomplishing more preheating, that is calcining, which permits treating a higher temperature solid in the kiln which can be designed to hold the material at a temperature for a predetermined time for controlled alkali elimination. The system herein to be described is materially different from the aforementioned grate burners in that cooler recoup air goes to grate beds as preheated combustion air for fuel in the pellets. It also can be directed to the grate bed as preheated combustion air for fuel in pellet beds, and it provides for controlled oxygen level that is required for burning bed fuel and the system can control the quantity of combustion air that is supplied to the air heaters. Also with the present concept bypass, cooler recoup air is utilized as tempering air, and the quantity that can be used is dependent upon the bypass and cooler exhaust temperatures. The cooler recoup air also is utilized in drying as supplementary drying heat. The present system also provides controlled cooler recoup air so that excess air is bypassed to waste gas systems to dump the excess air. The cooler recoup fan speed or damper for controlling kiln firing hood is a factor in varying flow. The damper for controlling the cooler recoup excess air bypass operates to stabilize air flow in the system by varying flow to the waste gas system.
Alternately, in a grate preheater kiln for processing fuel bearing (Kerogen) raw feed materials, the preburn is modified with an updraft ignition and combustion zone. This will make up that portion of the preheat zone required for processing the amount of raw feed combustibles. Its purpose is to ignite and continue the volatilization and combustion of the volatile combustile matter.